1. Field of the Invention
The present invention relates to a growth method and vapor phase growth apparatus for group III-V nitride semiconductors such as gallium nitride (GaN).
2. Related Background Art
conventionally known as a method of growing group III-V nitride semiconductors such as GaN are, for example, a hydride vapor phase epitaxy method (HVPE method) published in Japanese Patent Application Laid-Open No. HEI 10-215000 and an organic metal vapor phase epitaxy method (OMVFE method) published in Japanese Patent Application Laid-Open No. SHO 61-179527.
For growing gallium nitride (GaN) by the hydride vapor phase epitaxy method, (1) ammonia (NH3) as a material gas for nitrogen (N), (2) hydrogen chloride (HCl) for generating gallium chloride (GaCl) as a material gas for gallium (Ga) and (3) hydrogen (H2) as a carrier gas are continuously introduced into a reaction ampoule in which a boat containing Ga is disposed. AS GaCl, which is generated by a reaction between HCl and Ga, reacts with NH3, gallium nitride (GaN) grows on a seed crystal, According to this method, a large amount of material gases can be continuously supplied into the reaction ampoule, whereby the reaction rate can be improved as compared with the case using a so-called closed ampoule method in which no material gases are supplied from the outside.
For growing gallium nitride (GaN) by the organic metal vapor phase epitaxy method, (1) an organic metal such as trimethyl gallium (TMG) and (2) ammonia (NH3) are introduced as material gases into a reaction ampoule, whereas hydrogen or nitrogen is introduced therein as a carrier gas. AS TMG and NH3 react with each other, gallium nitride (GaN) grows on a seed crystal. According to this method, all the materials can be introduced into the reaction ampoule in the form of gas, whereby the film thickness can be controlled more precisely as compared with the hydride vapor epitaxy growth method.
However, the above-mentioned conventional hydride vapor epitaxy growth method and organic metal vapor phase epitaxy method have problems as follows. Namely, if group III-V compound semiconductors such as GaN are grown by the hydride vapor phase epitaxy method and organic metal vapor phase epitaxy method, then chlorine and hydrogen, which are no components of the group III-V compound semiconductors, will remain in the reaction ampoule as HCl, NH3, H2and the like, which are required to be let out of the reaction ampoule via an outlet. Namely, a so-called open ampoule method is employed in the hydride vapor epitaxy growth method and the organic metal vapor epitaxy growth method. As a consequence, most of the materials do not contribute to the growth and are discarded, whereby these methods are problematic in that the material efficiency is low. Also, for discarding a large amount of HCl, NE3, H2, and the like, a large-scale detoxification system is needed, which increases the cost. Namely, these methods are not suitable for making single crystals at a low cost.
In the so-called closed ampoule method, on the other hand, byproducts and the like are not let out, whereby the material efficiency is not so low as that in the hydride vapor epitaxy growth method and the organic metal vapor epitaxy growth method- However, while the growth rate has been required to improve in the field of making III-V compound semiconductors in recent years, no improvement in growth rate is expected in the closed ampoule method in which no material gases are supplied from the outside, since the amount of transportation of material gases is small.
In view of such circumstances, it is an object of the present invention to provide a group III-V nitride semiconductor growth method and vapor phase growth apparatus having a high material efficiency and a high growth rate.
In one aspect, the present invention provides a group III-V nitride semiconductor growth method for growing a group III-V nitride semiconductor on a seed crystal disposed within a reaction ampoule, the method comprising the steps of plasma-exciting nitrogen continuously introduced into the reaction ampoule and evaporating a group III element disposed within the reaction ampoule; and causing thus plasma-excited nitrogen and evaporated group III element to react with each other, so as to grow the III-V nitride semiconductor on the seed crystal.
In the group III-V nitride semiconductor growth method in accordance with this aspect of the present inventions nitrogen (N2) introduced into the reaction ampoule is excited so as to attain a plasma state, whereas a group III (group 3B) element such as gallium (Ga), for example, is evaporated within the reaction ampoule. As thus plasma-excited nitrogen and evaporated group III element react with each other, a group III-V nitride semiconductor such as gallium nitride (GaN)), for example, can be grown on the seed crystal. Here, since nitrogen is excited so as to attain a plasma state in this aspect of the present invention, it is more likely to react with the group III element as compared with a nitrogen molecule state in which the bonding strength between atoms is higher, and it can successively be introduced into the reaction ampoule unlike the case employing the closed ampoule method, whereby the growth rate of group III-V nitride semiconductor can be enhanced. Also, in this aspect of the present invention, only the group III element and nitrogen are used for growing the group III-V nitride semiconductor, and all the group III element and nitrogen contribute to growing the group III-V nitride semiconductor. Namely, no byproducts are generated upon growing the group III-V nitride semiconductor, whereby it is unnecessary to let out gases from within the reaction ampoule, whereby the material efficiency can be improved.
Preferably, in this aspect of the present invention, positive and negative pulsed voltages are alternately applied between two electrodes, so as to plasma-excite nitrogen between the electrodes.
In this case, since the positive and negative pulsed voltages are applied between the electrodes, an intermittent signal with a break between individual pulses is generated, whereby, as compared with the case where a continuous sine wave of high-frequency voltage is applied, the discharging phenomenon would not yield corona discharge, and nitrogen is more likely to be plasma-excited.
In another aspect, the present invention provides a group III-V nitride semiconductor growth method for growing a group III-V nitride semiconductor on a seed crystal disposed within a reaction ampoule, the method comprising the steps of causing nitrogen continuously introduced into the reaction ampoule to react with hydrogen within the reaction ampoule upon plasma excitation, so as to generate a hydride of nitrogen, and causing the hydride of nitrogen and a group III element evaporated within the reaction ampoule to react with each other, so as to grow the group III-V nitride semiconductor on the seed crystal; and then causing hydrogen generated upon growing the group III-V nitride semiconductor and nitrogen continuously introduced into the reaction ampoule to react with each other upon plasma excitation, so as to generate a hydride of nitrogen.
In the group III-V nitride semiconductor growth method in accordance with this aspect of the present invention, nitrogen continuously introduced into the reaction ampoule is caused to react with hydrogen within the reaction ampoule by plasma excitation, so as to generate a hydride of nitrogen such as NH, NH2, NH3, or the like. Within there action ampoule, on the other hand, a group all element such as gallium, for example, is evaporated. Then, the hydride of nitrogen and thus evaporated group III element react with each other, so that a group III-V nitride semiconductor such as gallium nitride grows on the seed crystal. Here, in this aspect of the present invention, since nitrogen diffuses into the vicinity of the seed crystal as a hydride such as NHx (X=1 to 3) and reacts with the group III element, it is more likely to react with the group III element as compared with a nitrogen molecule state in which the bonding strength between atoms is higher, and it can successively be introduced into the reaction ampoule by an amount equal to that required for the reaction unlike the case employing the closed ampoule method, whereby the growth rate of group III-V nitride semiconductor can be enhanced.
When a group III-V nitride semiconductor is grown upon the reaction between the hydride of nitrogen and the group III element, hydrogen which is no component of the group III-V nitride semiconductor is generated. Then, this hydrogen and nitrogen introduced into the reaction ampoule are caused to react with each other by plasma excitation, so as to generate again a hydride of nitrogen such as NH. Thereafter, this hydride of nitrogen and the evaporated group III element are caused to react with each other, whereby the group III-V nitride semiconductor can further be grown on the seed crystal. Namely, since hydrogen, which is no component of the group III-V nitride semiconductor, can repeatedly be utilized as being circulated within the reaction ampoule, it is unnecessary to let out gases from within the reaction ampoule, whereby the material efficiency can be improved in this aspect of the present invention.
Preferably, in this aspect of the present invention, positive and negative pulsed voltages are alternately applied between two electrodes, so as to cause nitrogen and hydrogen to react with each other upon plasma excitation between the electrodes.
In this case, since the positive and negative pulsed voltages are applied between the electrodes, an intermittent signal with a break between individual pulses is generated, whereby, as compared with the case where a continuous sine wave of high-frequency voltage is applied, the discharging phenomenon would not yield corona discharge, and nitrogen and hydrogen are more likely to react with each other upon plasma excitation.
In another aspect, the present invention provides a group III-V nitride semiconductor growth method for growing a group III-V nitride semiconductor on a seed crystal disposed within a reaction ampoule, the method comprising the steps of causing a group III element disposed within the reaction ampoule and a halogen molecule or halide to react with each other, so as to generate a halide of the group III element, and causing the halide of the group III element and plasma-excited nitrogen to react with each other, so as to grow the group III-V nitride semiconductor on the seed crystal; and then causing the halogen molecule or halide generated when growing the group III-V nitride semiconductor and the group III element disposed within the reaction ampoule to react with each other, so as to generate a halide of the group III element.
In the group III-V nitride semiconductor growth method in accordance with this aspect of the present invention, while nitrogen introduced into the reaction ampoule is excited so as to attain a plasma state, a group III element such as gallium disposed within the reaction ampoule and a halogen molecule such as Cl2 or a halide such as HCl are caused to react with each other, so as to generate a halide of the group III element such as gallium chloride (GaCl). As plasma-excited nitrogen and the halide of group III element are caused to react with each other, a group III-V nitride semiconductor such as gallium nitride, for example, can be grown on the seed crystal. Here, since nitrogen is excited so as to attain a plasma state, it is more likely to react with the group III element as compared with a nitrogen molecule state in which the bonding strength between atoms is higher, and it can successively be introduced into the reaction ampoule unlike the case employing the closed ampoule method, whereby the growth rate of group III-V nitride semiconductor can be enhanced. Further, since the group III element such as Ga is transported as a halide such as GaCl having a high equilibrium vapor pressure to the vicinity of the seed crystal, its transportation speed is faster than that in the case where the group III element is evaporated so as to reach the vicinity of the seed crystal, whereby the growth rate of group III-V nitride semiconductor can be enhanced.
When the group III-V nitride semiconductor is grown by the reaction between plasma-excited nitrogen and the halide of group III element, a halogen which is no component of the group III-V nitride semiconductor is generated as a halogen molecule or halide. Then, this halogen molecule or halide and the group III element such as gallium disposed within the reaction ampoule react with each other, so as to generate a halide of the group III element again, Thereafter, this halide of group III element and plasma-excited nitrogen can be caused to react with each other, so as to further grow the group III-V nitride semiconductor on the seed crystal. Namely, since a halogen, which is no component of the group III-V nitride semiconductor, can repeatedly be utilized as being circulated within the reaction ampoule, it is unnecessary to let out gases from within the reaction ampoule, whereby the material efficiency can be improved in this aspect of the present invention,
Preferably, in this aspect of the present invention, positive and negative pulsed voltages are alternately applied between two electrodes, so as to plasma-excite nitrogen between the electrodes.
In this case, since the positive and negative pulsed voltages are applied between the electrodes, an intermittent signal with a break between individual pulses is generated, whereby, as compared with the case where a continuous sine wave of high-frequency voltage is applied, the discharging phenomenon would not yield corona discharge, and nitrogen is more likely to be plasma-excited.
In another aspect, the present invention provides a group III-V nitride semiconductor growth method for growing a group III-V nitride semiconductor on a seed crystal disposed within a reaction ampoule, the method comprising the steps of causing nitrogen introduced into the reaction ampoule and hydrogen within the reaction ampoule to react with each other upon plasma excitation, so as to generate a hydride of nitrogen, and also causing a group III element disposed within the reaction ampoule and a halogen molecule or halide to react with each other, so as to generate a halide of the group III element, and causing the hydride of nitrogen and the halide of group III element to react with each other, so as to grow the group III-V nitride semiconductor on the seed crystal; and then causing the halogen molecule or halide generated upon growing the group III-V nitride semiconductor and the group III element disposed within the reaction ampoule to react with each other, so as to generate a halide of the group III element, and also causing hydrogen which is generated upon growing the group III-V nitride semiconductor and nitrogen to react with each other upon plasma excitation, so as to generate a hydride of nitrogen.
In the group III-V nitride semiconductor growth method in accordance with this aspect of the present invention, nitrogen introduced into the reaction ampoule and hydrogen within the reaction ampoule are caused to react with each other by plasma excitation, so as to generate a hydride of nitrogen such as NH, NH2, NH3, or the like, and also the group III element disposed within the reaction ampoule and a halogen molecule such as Cl2 or a halide such as HCl are caused to react with each other, so as to generate a halide of the group III element such as GaCl. Then, as the hydride of nitrogen and the halide of group III element are caused to react with each other, a group III-V nitride semiconductor such as gallium nitride, for example, can be grown on the seed crystal.
Here, since nitrogen diffuses to the vicinity of the seed crystal as a hydride and reacts with the group III element, it is more likely to react with the group III element as compared with a nitrogen molecule state in which the bonding strength between atoms is higher, and it can successively be introduced into the reaction ampoule by an amount equal to that required for the reaction unlike the case employing the closed ampoule method, whereby the growth rate of group III-V nitride semiconductor can be enhanced. Further, since the group III element such as Ga is transported as a halide such as GaCl having a high equilibrium vapor pressure to the vicinity of the seed crystal, its transportation speed becomes faster, whereby the growth rate of group III-V nitride semiconductor can be made faster than that in the case where the group III element is evaporated so as to reach the vicinity of the seed crystal.
When the group III-V nitride semiconductor is grown by the reaction between the hydride of nitrogen and the halide of group III element, hydrogen which is no component of the group III-V nitride semiconductor is generated, and also a halogen is generated as a halogen molecule or halide. Then, this hydrogen and nitrogen introduced into the reaction ampoule react with each other upon plasma excitation, so as to generate a hydride of nitrogen again, and also the halogen molecule or halide and the group III element such as gallium disposed within the reaction ampoule react with each other, so as to generate a halide of the group III element again. Thereafter, thus generated hydride of nitrogen and halide of group III element are caused to react with each other, whereby the group III-V nitride semiconductor can further be grown on the seed crystal. Namely, since hydrogen and halogen, which are no components of the group III-V nitride semiconductor, can repeatedly be utilized as being circulated within the reaction ampoule, it is unnecessary to let out gases from within the reaction ampoule, whereby the material efficiency can be improved in this aspect of the present invention.
Preferably, in this aspect of the present invention, positive and negative pulsed voltages are alternately applied between two electrodes, so as to cause nitrogen and hydrogen to react with each other upon plasma excitation between the electrodes.
In this case, since the positive and negative pulsed voltages are applied between the electrodes, an intermittent signal with a break between individual pulses is generated, whereby, as compared with the case where a continuous sine wave of high-frequency voltage is applied, the discharging phenomenon would not yield corona discharge, and nitrogen and hydrogen are more likely to react with each other upon plasma excitation.
Preferably, in the above-mentioned group III-V nitride semiconductor growth methods in accordance with the present invention, nitrogen is introduced into the reaction ampoule so as to keep a substantially constant total pressure within the reaction ampoule.
In this case, even when the partial pressure of nitrogen is lowered along with the growth of group III-V nitride semiconductor, nitrogen is introduced into the reaction ampoule so as to compensate therefore, whereby the group III-V nitride semiconductor can be grown stably.
In another aspect, the present invention provides a vapor phase growth apparatus for growing a group III-V nitride semiconductor, the apparatus comprising a reaction ampoule having a container disposed therein for containing a group III element and an inlet for introducing nitrogen, excitation means for plasma-exciting nitrogen introduced from the inlet, and heating means for heating a seed crystal disposed within the reaction ampoule and the container; wherein, upon growing the group III-V nitride semiconductor on the seed crystal, nitrogen is introduced from the inlet, and no gas is let out from within the reaction ampoule.
In the vapor phase growth apparatus in accordance with the present invention, nitrogen introduced from the inlet is excited by the excitation means so as to attain a plasma state. On the other hand, the group III element such as gallium contained in the container is evaporated by the heating means. Then, nitrogen in the plasma state and the evaporated group III element react with each other, so that a group III-V nitride semiconductor such as gallium nitride, for example, can be grown on the seed crystal, Here, since nitrogen is excited so as to attain a plasma state in this aspect of the present invention, it is more likely to react with the group III element as compared with a nitrogen molecule state in which the bonding strength between atoms is higher, and it can successively be introduced into the reaction ampoule unlike the case employing the closed ampoule method, whereby the growth rate of group III-V nitride semiconductor can be enhanced. Also, since the materials used in the growth apparatus in accordance with the present invention are only the group III element and nitrogen, which are components of the group III-V nitride semiconductor, the material efficiency can be improved. Further, while no gas is let out from within the reaction ampoule when growing the group III-V nitride semiconductor, all of nitrogen introduced into the reaction ampoule during the growth is used for growing GaN, whereby gases not contributing to the growth of GaN would be kept from remaining within the reaction ampoule in this aspect of the present invention.
When growing the group III-V nitride semiconductor in the growth apparatus in accordance with the present invention, a predetermined amount of hydrogen and halogen (halogen molecule such as Cl2 or halide such as HCl) may be introduced from the inlet. In this case, nitrogen introduced from the inlet into the reaction ampoule is plasma-excited by the excitation means, and further is caused to react with hydrogen, so as to generate a hydride of nitrogen such as NH, NH2, or NH3, and also the group III element and the halogen molecule or halide are caused to react with each other, so as to generate a halide of the group III element such as GaCl. Then, the hydride of nitrogen and the halide of group III element are caused to react with each other, whereby the group III-V nitride semiconductor such as gallium nitride, for example, can be grown on the seed crystal.
Here, since nitrogen diffuses into the vicinity of the seed crystal as a hydride such as NH and reacts with the group III element, it is more likely to react with the group III element as compared with a nitrogen molecule state in which the bonding strength between atoms is higher, and also, unlike the case employing the closed ampoule method, nitrogen is introduced into the reaction ampoule by an amount equal to that required for the reaction when growing the group III-V nitride semiconductor, whereby the growth rate can be enhanced. Further, since the group III element such as Ga is transported to the vicinity of the seed crystal as a halide such as GaCl, the growth rate of group III-V nitride semiconductor can be made faster than that in the case where the group III element is evaporated so as to reach the vicinity of the seed crystal.
When the group III-V nitride semiconductor is grown by the reaction between the hydride of nitrogen and the halide of group III element, hydrogen which is not component of the group III-V nitride semiconductor is generated, and also a halogen is generated as a halogen molecule or halide. Thus generated hydrogen and halogen molecule or halide would not be let out of the reaction ampoule when growing the group III-V nitride semiconductor. Then, hydrogen and nitrogen react with each other upon plasma excitation, so as to generate a hydride of nitrogen again, and also the halogen molecule or halide and the group III element such as gallium disposed within the reaction ampoule react with each other, so as to generate a halide of the group III element again. Thereafter, thus generated hydride of nitrogen and halide of group III element react with each other, whereby the group III-V nitride semiconductor further grows on the seed crystal. Namely, since hydrogen and the halogen, which are no components of the group III-V nitride semiconductor, can repeatedly be utilized as being circulated within the reaction ampoule, the material efficiency can be improved.
Preferably, in the vapor phase growth apparatus of the present invention, the excitation means has two electrodes, and a high-frequency power source for alternately applying positive and negative pulsed voltages between the electrodes.
In this case, since the high-frequency power source applies positive and negative pulsed voltages between the electrodes, an intermittent signal with a break between individual pulses is generated, whereby, as compared with the case where a continuous sine wave of high-frequency voltage is applied, the discharging phenomenon would not yield corona discharge, and nitrogen is more likely to be plasma-excited.